The present invention relates to a sensor function-equipped portable device for detecting physical information, such as water depth and altitude, and displaying such information or issuing a warning.
Although sensor function-equipped portable devices having a single function, such as dive computers, altimeters, and depth gauges, have been used in, for example, marine sports and mountaineering, sensor function-equipped electronic clocks have recently been manufactured that, in addition to their ordinary functions, e.g., basic time display function, alarm function, and timer function, also have sensor functions that use sensors to measure constantly changing physical information such as air pressure, water pressure, and temperature, and that display this information via a signal processor circuit; this kind of electronic clock has become more common.
With these sensor function-equipped portable devices, it is necessary to convert the physical information obtained as analog values into digital values in order to display the physical information detected by the sensors in a digital fashion; a 3 V high voltage power source means, for example, is required for this A/D conversion; in the past, a 3 V coin-type lithium cell has been used, or two or three 1.5 V button-type silver cells have been used.
However, a coin-type lithium cell or two or three buttontype silver cells result in a bulky clock element, which increases costs, so that for portable devices such as electronic clocks that have limited electronic circuit housing space and that must be inexpensive, operation with a single 1.5 V button-type silver cell is desired.
Prior art is described below with reference to accompanying drawing.
FIG. 2 is a block diagram of a conventional sensor signal processor used in a sensor-equipped portable device.
In FIG. 2, 101 is an air pressure sensor adapted to output an air pressure signal S1 proportional to an air pressure P, 102 is a sensor drive circuit adapted to drive the air pressure sensor 101 by causing constant current to flow in the air preasure sensor 101, 103 is an amplifier circuit that amplifies the air pressure signal S1 using an operational amplifier not shown in the figure, and that outputs the result as a signal S1xe2x80x2, 104 is an A/D converter circuit that subjects the signal S1xe2x80x2 output from the amplifier circuit 103 to A/D conversion and outputs the resulting product as data Dc, 105 is a sensor information data processor circuit that processes the data Dc and outputs the result as sensor information data Dj, 106 is a display unit that digitally displays the air pressure value on the basis of the sensor information data Dj output from the sensor information data processor circuit 105, 107 is a constant-voltage power source circuit that generates a xe2x88x922.6 V power source voltage Vreg, and 109 is a coin-type lithium cell that generates a xe2x88x923.0 V power source voltage Vss.
FIG. 7 is a diagram depicting the internal structure of the sensor drive circuit 102.
The sensor drive circuit 102 comprises a resistor 102a with a resistance value Rs and an operational amplifier 102b whose power source is a xe2x88x923.0 V power source voltage Vss. The negative input terminal of the operational amplifier 102b has the same potential Vs as the positive input terminal due to imaginary shortening with the air pressure sensor 101 as feedback resistance. A constant current Is, expressed by Formula (1) consequently flows in the resistor 102a, and the air pressure sensor 101 is thereby driven by the constant current Is.
Is=Vs/Rsxe2x80x83xe2x80x83(1)
FIG. 9 is a diagram depicting the internal structure of the constant-voltage power source circuit 107.
The constant-voltage power source circuit 107 comprises a constant-voltage generator 171 and a basic reference voltage generator 107a l composed of a resistor RO and a constant-current circuit 173. The constant-current circuit 173 allows a constant current Ir to flow through the resistor RO, so that a reference voltage Vr is generated due to the voltage drop across the resistor R0, and the reference voltage Vr is applied to the constant-voltage generator 171. The constant-voltage generator 171 subjects the reference voltage Vr to voltage/current amplification, and the resulting xe2x88x922.6 V power source voltage Vreg is supplied to the amplifier circuit 103 and the A/D converter circuit 104.
A conventional sensor signal processor having the aforementioned circuit structure operates as described below.
A voltage Vss of a coin-type lithium cell 109 serves as the power source, and when the air pressure sensor 101 is subjected to constant-current driving by the sensor drive circuit 102, an air pressure signal S1 proportional to the air pressure P applied to the air pressure sensor 101 is output. As shown in FIG. 10, the air pressure signal S1 is amplified by the amplifier circuit 103, with a voltage of Vreg/2 that is half of the power source voltage Vreg that serves as the reference, resulting in a signal S1xe2x80x2. As far as this amplified signal S1xe2x80x2 is concerned, the difference between the voltage Vreg/2 and the signal S1xe2x80x2 is subjected to digital conversion by the A/D converter circuit 104, with the voltage Vreg/2 serving as the reference, to produce digital data Dc. The digital data Dc is converted into a sensor information signal Dj by the sensor information processor circuit 105, and the display unit 106 displays the air pressure value (e.g., 1013 hPa) based on this sensor information signal Dj. The signal S1xe2x80x2 that has been amplified by the amplifier circuit 103 varies within a range between the voltage Vreg/2 and the voltage Vreg shown in FIG. 10, the potential difference between Vreg and Vreg/2 is taken as the dynamic range, and, for a given air pressure range, the resolution of the A/D converter circuit 104 can be increased for a larger dynamic range, so that the air pressure value display resolution can be increased. Since the number of bits per unit display air pressure can be increased, it is also possible to reduce the variation in the air pressure value display that is caused by bit errors due to poor A/D conversion reproducibility.
As described above, when display resolution and bit errors during A/D conversion are taken into account, it is sometimes necessary to increase the dynamic range of the signal S1xe2x80x2 amplified by the amplifier circuit 103. For this reason, the power source voltage Vreg must be about xe2x88x922.6 V to generate such a Vreg, and the constant-voltage power source circuit 107 must have a power source voltage Vss that is xe2x88x923.0 V or less, and the cell 109 must be of a voltage of 3 V or more.
To maintain a power source voltage of 3 V or more, however, either a coin-type lithium cell with a large diameter or a plurality of 1.5 V button-type silver cells must be used; as far as portable devices such as electronic clocks with limited electronic circuit element housing space are concerned, the size of the module becomes considerable, and this is disadvantageous in terms of design and cost.
The present invention was devised in light of the aforementioned situation, and its objective is to provide a sensor function-equipped portable device that can maintain A/D conversion resolution and reproducibility using only a single small and inexpensive 1.5 V button-type silver cell.
To achieve this objective, the present invention provides a sensor function-equipped portable device comprising a sensor for detecting physical information, a sensor drive circuit for driving the sensor, an amplifier circuit for amplifying the sensor signal from the sensor, an A/D converter circuit for converting the output signal of the amplifier circuit into digital information, a sensor information data processor circuit for preparing sensor information data from the digital information output from the A/D converter circuit, and a display unit for displaying physical values based on the sensor information data from the sensor information data processor circuit, wherein a low voltage power source cell and a step-up power source circuit for elevating the low voltage of the cell to a high voltage are furnished, and the sensor drive circuit is directly driven by the low voltage of the cell, and the amplifier circuit and the A/D converter circuit are driven by the high voltage that has been elevated by the step-up power source circuit.
The sensor function-equipped portable device according to the present invention further comprises a constant-voltage power source circuit for stabilizing the high voltage that is elevated by the step-up power source circuit, and in which the amplifier circuit and the A/D converter circuit are driven by the high voltage that has been stabilized by the constant-voltage power source circuit.